French National Research Institute for Agriculture, Food, and the Environment (INRAE)

The French National Research Institute for Agriculture, Food, and the Environment (INRAE) is a public research establishment. It is a community of 12,000 people with more than 200 research units and 42 experimental units located throughout France. The institute is among the world leaders in agricultural and food sciences, in plant and animal sciences, and is 11th in the world in ecology and environment. INRAE’s main goal is to be a key player in the transitions necessary to address major global challenges. In the face of the increase in population, climate change, scarcity of resources and decline in biodiversity, the institute develops solutions for multiperformance agriculture, high quality food and sustainable management of resources and ecosystems.

By joining our teams, you benefit from (depending on the type of contract):
- until 30 days of annual leave + 15 days “Reduction of Working Time” (for a full time);
- parenting support: CESU childcare, leisure services;
- skills development systems: training, career advise;
- social support: advice and listening, social assistance and loans;
- holiday and leisure services: holiday vouchers, accommodation at preferential rates;
- sports and cultural activities;
- collective catering.

Homepage: https://www.inrae.fr/en

Project Description
Leaf phenology, the study of the drivers and dynamics of leaf budburst, unfolding, maturation and senescence, controls carbon, water
and energy exchanges between forests and the atmosphere. Current approaches establish an empirical link between leaf phenology and
standardised air temperature (the macroclimate), and fail to capture the true conditions sensed by trees (the microclimate). This results in
a biased interpretation of the role of other factors, such as light, as well as the response of trees to climate warming, and leads to major
uncertainties in the projections of forest dynamics and resilience.

The aim of this thesis project is to fill this gap by quantifying the role of microclimate on leaf phenology. By combining in situ
observations, controlled experiments and modelling, this project will 1) quantify the microclimatic control of leaf phenology and clearly
disentangle the factors that govern it, 2) develop new phenology models based on bud and leaf temperature and 3) reassess the
sensitivity of phenology to past and future climate across scales.

Context
The phenology of leaves, i.e. their development, growth and senescence, has a direct influence on forest productivity and biomass. It also influences local weather conditions and long-term climate through transpiration, albedo and carbon storage (Peñuelas et al. 2009). The increasing rate of climate change is translating into phenological shifts on a global scale, increasing the risks (e.g. of frost or spring drought) faced by trees that were adapted to past environmental conditions (Peaucelle et al. 2019). Despite centuries of research and observations, a thorough fundamental understanding of the environmental drivers of leaf phenology is still lacking.

The poor representation of phenology in terrestrial biosphere models is considered one of the main uncertainties in carbon cycle estimates and future climate forecasts. Temperature is recognized as the main driver of leaf phenology in extra-tropical ecosystems. For this reason, phenological models generally rely on metrics based on the air temperature preceding phenological events, often in the form of a sum of degree-days. Numerous variants of phenological models have been proposed to describe budburst, growth and leaf senescence, including sunshine duration or photoperiod as another climatic variable, but with often limited success compared to a model based solely on growing degree-days. One of the reasons for these limitations is that today’s climate is evolving too rapidly, so that models based on past, relatively “stable” climatic conditions are unable to accurately predict phenology in recent years, which have seen a number of record-breaking climatic extremes. Another reason is that these models are based on the average air temperature of a geographical location, also known as “macro-climatic” temperature, which does not allow us to capture the spatio-temporal variability in the landscape of organ temperatures (buds, leaves), and therefore phenology.

This thesis aims to study in greater detail the biophysical processes, and in particular thermal processes, that control leaf phenology. Recent studies have highlighted the key role of solar radiation and biophysical properties such as bud and leaf albedo in spring phenology (Vitasse et al., 2021; Peaucelle et al. 2022), as well as in physiological activity, the latter being identified as a potential driver of leaf senescence. This suggests that the temperature conditions perceived by tree buds and leaves (i.e. the microclimate) are of paramount importance for understanding and accurately simulating leaf phenology and their response to future environmental conditions.

Objectives
The general aim of this thesis project is to study the role of microclimate on leaf phenology in forests. More specifically, you will focus on European deciduous forest species for which microclimatic processes due to seasonal canopy closure are particularly pronounced (De Frenne et al. 2021), and you will seek to test the following two hypotheses:
(H1) The energy balance of buds or leaves is essential to disentangle the effects of temperature, radiation and photoperiod on leaf phenology.
(H2) Photoperiod-sensitive deciduous species perceive light through the photosynthetic tissues of buds or bark.

Methods
To meet these objectives, you will develop a multidisciplinary approach combining experiments under controlled conditions, field observations, data analysis and process modeling through 3 tasks:
1) Analyze the relationships between organ temperature (buds, leaves), phenology and environmental conditions. For this task, you will be required to carry out several controlled experiments (e.g. temperature, sunshine, irrigation, fertilization) and to install sensors (e.g. thermal camera) and carry out measurements (e.g. functional traits, photosynthesis) in the field.
2) Develop and calibrate a phenology model that takes into account bud and leaf temperature and the effect of microclimate. This task will involve evaluating existing models and developing a new phenology model based on the results of Task 1.
3) Reassess the sensitivity of past and future leaf phenology to climatic conditions. The aim of this task will be to reanalyze past phenology data and make future predictions at several spatial and temporal scales using the phenology model developed in Task 2. This new model could be coupled to other processes in order to quantify the role of phenology on associated biogeochemical cycles in the context of global change.

References
De Frenne P, Lenoir J, Luoto M, et al (2021) Forest microclimates and climate change: Importance, drivers and future research agenda.
Global Change Biology 27:2279–2297. https://doi.org/10.1111/gcb.15569.

Peaucelle M, Peñuelas J, Verbeeck H (2022) Accurate phenology analyses require bud traits and energy budgets. Nature Plants 8:915–
922. https://doi.org/10.1038/s41477-022-01209-8.

Peaucelle M, Janssens IA, Stocker BD, Descals Ferrando A, Fu YH, Molowny-Horas R, Ciais P, Peñuelas J (2019) Spatial variance of
spring phenology in temperate deciduous forests is constrained by background climatic conditions. Nature communications, 10, 5388.

Peñuelas J, Rutishauser T, Filella I (2009) Ecology. Phenology feedbacks on climate change. Science (New York, N.Y.), 324, 887–888.

Vitasse Y, Baumgarten F, Zohner CM, et al (2021) Impact of microclimatic conditions and resource availability on spring and autumn
phenology of temperate tree seedlings. New Phytologist 232:537–550. https://doi.org/10.1111/nph.17606.

Contact Marc Peaucelle (marc.peaucelle@inrae.fr) and Jérôme Ogée (jerome.ogee@inrae.fr).

Profile and skills required
Recommended training: Master 2 or equivalent in environmental sciences.
Desired knowledge : Computer programming (R, Python), environmental data analysis.
Experience: Theoretical or experimental research.
Skills required: Ability to work in a team, to write (scientific articles) and to express oneself (scientific conferences) in English. Keen
interest in forest fieldwork, experimentation and modeling.